1. Field of the Invention
The present invention relates to an etching method and an etched product, such as a quartz wafer, that is formed by the etching method. More specifically, the present invention concerns a method for providing a highly efficient forming process and for producing a high-quality formed product.
2. Description of the Related Art
Along with the developments of communication apparatuses having higher frequencies and microcomputers having higher operation frequencies, there have been ever-increasing demands for piezoelectric vibration devices such as quartz resonators and quartz filters having high frequencies. In general, with respect to quartz wafers (quartz plates) that have achieved high frequencies, the thickness sliding vibration of an AT-cut quartz plate has been often utilized. Further, as is well-known, the frequency is determined by the thickness, and the frequency is inversely proportional to the thickness. For example, in an attempt to obtain 600 MHz as a basic vibration frequency, it is necessary to provide an ultra-thin piezoelectric vibration plate having a thickness of not more than 3 μm. With respect to a machining process of such an ultra-thin plate, a difficult polishing process is required, making it difficult to improve the production yield.
In order to solve these problems, a so-called reversed mesa structure has been proposed, in which, as shown in FIG. 51, a recessed section 101 is formed in the center portion of a quartz wafer 100, a vibration area 102 that has been machined into a thin shape is placed on the bottom of this recessed section 101, and the vibration area 102 is reinforced by a thick reinforcing portion 103 that is formed on the periphery thereof. The quartz vibration plate of this type has a structure in which the quartz wafer 100 having the thin vibration area 102 and the reinforcing portion 103 formed on the periphery thereof is provided with an exciter electrode and a drawing electrode (not shown). The application of this structure makes the vibration area 102 considerably thinner than that of a conventional vibration area, and also increases the yield. The quartz wafer of this type is disclosed in, for example, Japanese Patent Application Laid-open No. 2000-341064.
Moreover, with respect to one type of the quartz vibration plate 1 of this reversed mesa structure, another structure has been well-known, in which, as shown in a cross-sectional view of FIG. 52(g), a step portion 104 having a step shape is formed between the vibration area 102 and the reinforcing portion 103 so that the mechanical strength of the quartz wafer 100 is improved and it is possible to prevent an external force from transmitting to the vibration area 102. The following description will discuss forming operations of the reversed mesa type quartz vibration plate having this step portion 104.
As shown in FIG. 52(a) (a drawing that shows cross-sections of a quartz wafer and a mask layer), on a quartz wafer a1 having upper and lower faces that have been formed into mirror faces through a polishing process, mask layers (resist film) RR are formed over the entire lower face and over one portion of the upper face. These mask layers RR have a two-layer structure of, for example, chromium (Cr) and gold (Au). Moreover, the formation area of the mask layer RR on the upper face covers the entire area except for a portion in which the above-mentioned vibration area 102 is to be formed. More specifically, the mask layer RR is formed over the entire upper face of the quartz wafer a1, and the mask layer RR which is located on a position corresponding to the vibration area 102 is selectively removed through a photolithography technique, etc. Further, the quartz wafer a1 is immersed into an etching solution, such as hydrofluoric acid+ammonium fluoride, with the mask layer RR which is left on the quartz wafer a1 being used as a mask; thus, a first wet etching process is carried out. FIG. 52(b) shows a state in which this first wet etching process has been completed. Thus, the first step portion e1 is formed.
Next, one portion of the remaining mask layer RR is further removed selectively. As shown in FIG. 52(c), this removal area of the mask layer RR corresponds to an area on which a step portion e2 is to be formed as a second step. Thereafter, a second etching process is carried out by an etching solution in the same manner as described above. FIG. 52(d) shows a state in which the second wet etching process has been completed. Thus, the second step portion e2 is formed.
Moreover, one portion of the remaining mask layer RR is selectively removed. As shown in FIG. 52(e), this removal area of the mask layer RR corresponds to an area on which a step portion e3 is to be formed as a third step. Thereafter, a third etching process is carried out by an etching solution in the same manner as described above. FIG. 52(f) shows a state in which the third wet etching process has been completed. Thus, the third step portion e3 is formed.
In this manner, after having been subjected to a plurality of etching processes, all the mask layers RR on the upper and lower faces are removed so that, as shown in FIG. 52(g), a quartz wafer 100, which has a step portion 104 having a step shape between the vibration area 102 and the reinforcing portion 103, is formed. Then, predetermined electrodes are formed on the upper and lower faces of the vibration area 102 so that a quartz resonator is manufactured.
Moreover, as shown in FIG. 53(g), a quartz wafer of the so-called mesa type, which has a quartz wafer 100 having a thickness dimension in the center portion which is greater than the thickness dimension of the peripheral edge portion, is also formed through virtually the same etching method as described above. In other words, as shown in FIG. 53(a), on a quartz wafer a1 having upper and lower faces that have been formed into mirror faces through a polishing process, mask layers RR are formed over only the portions of upper and lower faces except for the peripheral edges thereof. Further, the quartz wafer a1 is immersed into an etching solution, such as hydrofluoric acid+ammonium fluoride, with the mask layer RR serving as a mask; thus, a first wet etching process is carried out. FIG. 53(b) shows a state in which this first wet etching process has been completed.
Next, as shown in FIG. 53(c), outer edge portions of the remaining mask layers RR are removed. Then, the second wet etching process is carried out in the same manner as described above by using an etching solution. FIG. 53(d) shows a state in which the second wet etching process has been completed.
Moreover, as shown in FIG. 53(e), the outer edge portions of the remaining mask layers RR are removed. Thereafter, a third etching process is carried out by an etching solution in the same manner as described above. FIG. 53(f) shows a state in which the third wet etching process has been completed.
In this manner, after having been subjected to a plurality of etching processes, all the mask layers on the upper and lower faces are removed so that, as shown in FIG. 53(g), a quartz wafer 100 is formed in which the thickness dimension of the center portion of the quartz wafer 100 is set to be greater than the thickness dimension of the outer edge portion thereof with a step portion 104 having a step shape being placed between the center portion and the outer edge portion.
However, in the above-mentioned quartz wafer forming operations, a plurality of etching processes are required, and an immersing process of the quartz wafer into the etching solution and a drying process of this quartz wafer need to be repeated a plurality of times. For this reason, complex jobs are required, while the working time is prolonged, and repeated immersing processes and drying processes might cause surface roughness on the quartz wafer.
One of the reasons for this surface roughness is that dust or the like adheres to the surface of the vibration area in the above-mentioned drying process and that the sequence proceeds to the immersing process with the dust or the like still adhering thereto. In the event of such surface roughness, adverse effects (such as deviations in the reference frequency) might be given to the performance of the quartz resonator, especially, to that of the reversed mesa type. Moreover, in the immersing process after the drying process, air might remain in the vicinity of the step portion that has been formed, and in such a case, the etching solution is not allowed to reach the vicinity of the step portion, thereby resulting in an etching failure and the subsequent failure of not forming a quartz wafer into a predetermined shape. Moreover, at the final stage of a plurality of immersing processes, the quartz wafer tends to have a portion whose thickness is considerably thin, where, as a result, damages such as cracking occur at this thin portion and the peripheral portion thereof in the immersing process and the drying process, thereby causing degradation in the yield.
Moreover, the above-mentioned problem of surface roughness and degradation in the yield might occur not only in quartz wafers, but also in glass, metal, semiconductors and the like when these materials are subjected to similar etching processes.
Furthermore, tuning-fork type quartz resonators, which can be easily miniaturized, have been conventionally known as one type of piezoelectric vibration device. For example, as has been disclosed in Japanese Patent Application Laid-open No. H10-294631, the resonator of this type is provided with a tuning-fork-type quartz vibration member having an arrangement in which a quartz wafer, which is formed into a tuning-fork shape through an etching process, is provided with predetermined electrodes that are formed on the surface thereof through a photolithographic technique.
Japanese Patent Application Laid-open No. 2002-76806 has disclosed an arrangement in which grooves are formed in center portions of the surface and rear surface (main faces) of each of the leg portions of the tuning-fork-type quartz vibration member. This structure having grooves on the surface and rear surface of each of the leg portions is effective since it is possible to reduce the vibration loss in each leg portion even when the vibration member is miniaturized, and to suppress the CI (crystal impedance) value to a low level. The tuning-fork-type quartz resonator of this type is well suited for use in precision instruments such as watches.
The following description will discuss processes which are disclosed in the above-mentioned official gazette as a method of forming a tuning-fork-type quartz wafer that is provided with grooves on the surface and rear surface of the above-mentioned leg portions.
First, as shown in FIG. 54(a), a quartz substrate a, which is a quartz plate, is machined into a plate shape. In this case, the surface and rear surface of the quartz substrate a are polished into mirror surfaces.
Next, as shown in FIG. 54(b), a Cr (chromium) film b1 is vapor-deposited on the surface and rear surface of the quartz plate a, and an Au (gold) film b2 is further vapor-deposited thereon by using a sputtering device (not shown). Further, as shown in FIG. 54(c), photoresist layers c are formed on the surfaces of the metal films b1, b2 thus formed.
The photoresist layer c is partially removed so that a vibration member forming area d that is coincident with a shape (tuning-fork shape) of a tuning-fork-shape quartz wafer to be formed and frame portions e that are outer edges of the quartz substrate a are formed; thus, an outer-shape patterning process is carried out. FIG. 54(d) shows a cross-section in this state, and FIG. 55(a) shows a perspective view thereof. As shown in FIG. 55(a), in this state, the photoresist layers c are formed in such a manner so that the predetermined shape of the tuning-fork-type quartz wafer appears thereon.
Thereafter, as shown in FIG. 54(e), respective metal films b1, b2, which correspond to portions at which no photoresist layer c is formed in FIG. 54(d), are removed by an Au etching solution and a Cr etching solution. Therefore, as shown in FIG. 55(b), the quartz substrate a is exposed to the portions from which the respective metal films b1, b2 have been removed.
Next, as shown in FIG. 54(f), all the photoresist layers c remaining as shown in FIG. 54(e) are removed.
Thereafter, as shown in FIG. 54(g), photoresist layers f are formed over the entire surface and rear surface of the quartz substrate a.
Further, as shown in FIG. 54(h), one portion of each photoresist layer f is removed. More specifically, a groove patterning process is carried out so that not only the photoresist layers f corresponding to portions other than the vibration member forming area d and the frame portion e, but also the portions of the photoresist layers f corresponding to grooves g (FIG. 54(l)), are removed.
Next, as shown in FIG. 54(i), an outer-shape etching process is carried out by using a quartz etching solution. In other words, the outer-shape etching process is carried out with only the vibration member forming area d and the frame portion e which are left.
Successively, as shown in FIG. 54(j), portions of the respective metal films b1, b2 that correspond to the grooves g to be formed on the leg portions of the tuning-fork-shape quartz wafer are removed by an Au etching solution and a Cr etching solution.
Moreover, as shown in FIG. 54(k), the quartz substrate a is etched to a predetermined depth by a quartz etching solution so that grooves g are formed on the respective faces of each leg portion with its cross-section having a virtually H-letter shape. Then, the photoresist layer f and the respective metal films b1, b2 are removed so that a tuning-fork-type quartz wafer h having leg portions is formed in which each of the leg portions has a cross-section having a virtually H-letter shape as shown in FIG. 54(l).
With respect to the tuning-fork-type quartz wafer h formed as described above, predetermined electrodes are formed on the upper and lower faces of the vibration area so that a tuning-fork-type quartz vibration member is manufactured, and this tuning-fork-type quartz vibration member is attached to a package so that a tuning-fork-type quartz resonator is completed.
In the forming method disclosed by the above-mentioned official gazette, after the outer-shape etching process (process of FIG. 54(i)) for removing an area which is located outside of the outer edge of a tuning-fork-type quartz wafer h to be formed has been first carried out, the groove etching process (process of FIG. 54(k)) for forming grooves g on the main faces of each leg portion is carried out. In other words, the outer-shape forming process and the groove forming process of the quartz wafer h are carried out through individually separated processes.
For this reason, this conventional technique requires an increased number of processing operations, which results in problems of complex processing operations and prolonged processing time. Moreover, etching processes using the quartz etching solution are carried out in the respective outer-shape forming process and groove forming process. Therefore, at least the quartz etching processes of two times need to be carried out, and as a result, that problems such as surface roughness of the quartz wafer might occur.
In the case when the grooves are formed on the surface and rear surface of each leg portion by using the method disclosed in the above-mentioned gazette, extremely high processing precision is required for these grooves. This is because the structure having the above-mentioned grooves tends to have greater deviations in the vibration frequency in comparison with those having no grooves. In order to reduce these deviations, one of the effective methods is to carry out the processing of these grooves with high precision.
Moreover, the structure in which these grooves are formed makes it possible to reduce the CI value to a low level, and in order to effectively reduce the CI value, it is necessary to carry out the processing of the grooves with high precision.
The present invention has been devised to solve the above-mentioned problems with the conventional technique, and its first objective is to provide an etching method which can form a quartz wafer having a predetermined shape (for example, a shape having the above-mentioned step portion) by using only one etching process so that it is possible to prevent surface roughness of the quartz wafer, etching failure and damages to a thin portion and the peripheral portion thereof, and also to improve the processing precision of the etched product, as well as forming such an etched product.
Moreover, the above-mentioned quartz wafer is housed in a package which is made from ceramics such as alumina, and is secured to the inside of the package by a bonding agent. In other words, drawing electrodes are bonded to terminals inside the package by a conductive bonding agent so that the quartz wafer is connected to the package electrically as well as mechanically. Thus, a quartz resonator of, for example, a surface packaging type is manufactured.
In this case, since the above-mentioned quartz wafer of the reversed mesa type or the flat-plate-shape quartz wafer of the thin-film type is secured into the package, the vibration area of the quartz wafer is subjected to the influences of stress which is exerted through curing shrinkage of the bonding agent. This causes variations in the resonance frequency of the quartz wafer, thereby resulting in a high possibility of failure in obtaining predetermined frequency characteristics. Moreover, there is a high possibility that an external force, which is applied from the outside of the package, is directly exerted on the vibration area of the quartz wafer, and this case also causes a high possibility of failure in obtaining predetermined frequency characteristics.
Moreover, when an attempt is made to secure the quartz wafer into the package in a manner so as to not receive the influences of curing shrinkage of the bonding agent, the bonding agent needs to be applied to a position that makes the quartz wafer less susceptible to the influences of curing shrinkage with high precision, and this requires a manufacturing device with high performances, which results in high costs and complex manufacturing processes and the subsequent degradation in the processing efficiency.
The present invention has been devised to solve these problems, and its second objective is to provide an etching method which makes it possible to prevent influences, etc. of curing shrinkage of a bonding agent at the time of securing the piezoelectric vibration device to the package through the bonding agent from reaching the vibration area, and consequently to obtain preferable frequency characteristics of the piezoelectric vibration device, and an etched product which is obtained through such a method.
Moreover, conventionally, a tuning-fork-type quartz resonator which can be easily miniaturized has been known as one type of a piezoelectric vibration device. For example, as disclosed in Japanese Patent Application Laid-open No. H10-294631, the resonator of this type is provided with a tuning-fork-type quartz vibration member having an arrangement in which a quartz wafer, which is formed into a tuning-fork shape through an etching process, is provided with predetermined electrodes that are formed on the surface thereof through a photolithographic technique. The following description will discuss the forming processes of these electrodes.
FIG. 39 is a front view that shows a generally-used tuning-fork-type quartz vibration member 10, and electrode-forming portions are indicated by slanting lines. FIG. 40 is a drawing that shows processes in which electrodes 73, 74 are formed on the surface of a quartz wafer 1A through the photolithographic technique, and is a cross-sectional view taken along line II-II of FIG. 39.
In the processes of forming the electrodes 73, 74, first, with respect to a quartz wafer 1A (FIG. 40(a)) which is formed into the above-mentioned tuning-fork shape, an electrode film 15, which is made of a material such as chromium or gold, is formed on the entire surface of the quartz wafer 1A through a vacuum vapor deposition method or the like (FIG. 40(b)). Then, the entire surface of the quartz wafer 1A is coated with a resist film 31 which is made from a positive-working-type photoresist solution (FIG. 40(c)). This resist film 31 is subjected to predetermined exposing and developing processes so that opening sections 75 are formed on the resist film 31 at areas to be etched to form electrode films 15 (FIG. 40(d)). The electrode films 15 which are exposed to these opening sections 75 are etched so that the electrode film 15 (FIG. 40(e)) is partially removed, and the above-mentioned resist film 31 is then removed (FIG. 40(f)). Thus, electrodes 73, 74 are formed on only predetermined areas on the quartz wafer 1A so that a tuning-fork-type quartz vibration member 10 is obtained.
With respect to the electrode forming areas of the tuning-fork-type liquid crystal vibration member 10 thus formed, as shown in FIG. 39 and FIG. 40(f), a continuous area, which is formed over the two faces that are adjacent to (orthogonal) each other through respective edge portions of the quartz wafer 1A, is prepared. This arrangement is made so that an electrode 73 (74) of the main face 61a (62a) in one of the leg portions 61 (62) is connected to an electrode 73 (74) of the side face 62b (61b) in the other leg portion 62 (61) so as to conduct to each other. For this reason, with respect to the piezoelectric vibration device of this type, it is very important to ensure the continuity of electrodes 73, 73 (74, 74) of the respective edge portions.
However, the above-mentioned processes of forming the electrodes have the following problem. FIG. 56, which corresponds to FIG. 40, is a drawing that explains the problem. In general, in the case when the entire surface of the quartz wafer 1A is coated with a resist film 31 as shown in FIG. 56(c), the quartz wafer 1A is immersed into a resist solution vessel, or the resist solution is applied onto the quartz wafer 1A by using a spray. In this case, surface tension is exerted on the resist solution which is applied on each of the faces of the quartz wafer 1A so that the resist solution is drawn in directions that are indicated by arrows of broken lines as shown in FIG. 56(c). In other words, the resist solution is allowed to easily flow in directions departing from the edge portions. For this reason, the amount of application of the resist solution becomes insufficient at the edge portions, and in some cases, no resist solution exists on the periphery of an edge portion. FIG. 57 shows a front view of the quartz wafer 1A in a state where no resist solution (indicated by an imaginary line in the drawing) exists at an edge portion due to the influences of the surface tension.
In the case when the above-mentioned exposing and developing processes and etching processes of the electrode film 15 are carried out in the state in which no resist solution is located on the periphery of an edge portion, as shown in FIGS. 56(e) and 56(f), not only is the electrode film 15 that is exposed to the opening sections 75 removed, but the electrode film 15 on the periphery of the edge is also removed, with the result being that it is not possible to ensure the continuity of the electrodes 73, 74 at the edge portion, and consequently, a defective quartz vibration member 1 is produced.
In order to solve this problem, a method has been proposed in which the amount of application of the resist solution to the quartz wafer 1A is increased.
However, even with this method, it is not possible to avoid the generation of the above-mentioned surface tension. For this reason, although the resist film 31 having a certain degree of film thickness is allowed to exist on the periphery of the edge, the film thickness of the resist film 31 becomes unnecessarily great in the other portions. This causes an insufficient amount of exposing energy to these portions having the great film thickness, which results in a failure to sufficiently carry out exposing and developing processes on these portions. Consequently, the resist film 31 partially remains at areas (areas in which the above-mentioned opening sections 75 are to be formed) from which the resist film 31 needs to be removed, which results in a possibility that etching is not carried out at necessary portions of the electrode film 15.
Moreover, another method has been proposed in which the amount of exposure to the portions of the resist film 31 having a great film thickness is set to a greater value so as to not leave unnecessary resist film 31. However, this method causes degradation in the patterning precision, which results in a failure to miniaturize the resonator.
The present invention has been devised to solve this problem, and its third objective is to solve the problem caused by the surface tension which is exerted in the resist solution that is applied to the respective surfaces of the quartz wafer (piezoelectric vibration substrate), to reduce the rate of generation of defective products and, consequently, to improve the productivity of the piezoelectric vibration device.